feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/github.com/emirpasic/gods/lists/lists.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2015, Emir Pasic. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package lists provides an abstract List interface.
+//
+// In computer science, a list or sequence is an abstract data type that represents an ordered sequence of values, where the same value may occur more than once. An instance of a list is a computer representation of the mathematical concept of a finite sequence; the (potentially) infinite analog of a list is a stream.  Lists are a basic example of containers, as they contain other values. If the same value occurs multiple times, each occurrence is considered a distinct item.
+//
+// Reference: https://en.wikipedia.org/wiki/List_%28abstract_data_type%29
+package lists
+
+import (
+	"github.com/emirpasic/gods/containers"
+	"github.com/emirpasic/gods/utils"
+)
+
+// List interface that all lists implement
+type List interface {
+	Get(index int) (interface{}, bool)
+	Remove(index int)
+	Add(values ...interface{})
+	Contains(values ...interface{}) bool
+	Sort(comparator utils.Comparator)
+	Swap(index1, index2 int)
+	Insert(index int, values ...interface{})
+	Set(index int, value interface{})
+
+	containers.Container
+	// Empty() bool
+	// Size() int
+	// Clear()
+	// Values() []interface{}
+	// String() string
+}